This invention relates generally to a method and apparatus for detecting trace contaminants in a gas stream and, more specifically, detecting the presence of moisture in a corrosive gas atmosphere using acoustic wave (AW) devices.
At the present time, there is no commercially available solid-state in-line sensor system to detect trace moisture (H.sub.2 O) in a corrosive atmosphere. Corrosive gases are widely used in a variety of industrial applications, such as semiconductor and pharmaceutical manufacturing. Trace contaminants, like moisture (H.sub.2 O), in corrosive process gases lead to diminished yields and increased maintenance costs as a result of particle formation and corrosion of equipment. Particle formation is believed to occur as a result of moisture levels as low as 10 ppb in corrosive gases. The obvious solution to the contamination problem is to prevent moisture from being present in the gas stream and to detect moisture upsets during actual gas use. Known ways of preventing H.sub.2 O contamination include procedures such as extensive purges, evacuation of plumbing networks, and use of corrosion resistant plumbing networks. However, it is well known in the art that corrosive gases which come in compressed liquid form, such as hydrogen halides like hydrochloric acid (HCl) and hydrogen bromide (HBr), have a time varying H.sub.2 O output. As an example, H.sub.2 O in the vapor phase from a single tank of compressed liquid HCl can vary from 20 ppm to over 70 ppm as it is consumed. The liquid phase equilibrium constant of the reaction of H.sub.2 O in liquid phase HCl can be as much as three orders of magnitude larger than the equilibrium constant for vapor phase HCl. As an example, as the HCl is consumed, the relative concentration of liquid phase H.sub.2 O increases due to its lower vapor pressure; the result is an increase in the liquid concentration of H.sub.2 O in the HCl which then increases the vapor phase concentration of H.sub.2 O in the HCl.
Hence, unacceptable levels of H.sub.2 O can be introduced from the HCl gas tank even after careful drying of the distribution system. This demonstrates the need for a low cost, solid-state, in-line sensor system. Although there exist techniques that can detect moisture in a gas stream, such as Karl Fischer titration, infrared spectrometry, and gravimetric procedures using desiccants, there is commercially only one H.sub.2 O detection system that will work in a corrosive environment like HCl or HBr. This method of detecting H.sub.2 O in a corrosive gas is based on the common chilled mirror dew point/frost point hygrometer. A frost point hygrometer works by chilling the mirror and monitoring its reflectivity as a function of time. By calibrating the apparatus to determine frost formation temperature versus concentration, the H.sub.2 O concentration in an unknown sample of the corrosive gas can be determined.
There are several disadvantages to using a frost point hygrometer: first, the moisture concentration measurement requires large volumes of gas to flow across the mirror, at a minimum 1 liter/min. Further, a frost point hygrometer is inherently inaccurate at very low moisture levels (&lt;1 ppm) because of the low rate at which moisture is transported to and condenses on the mirror. Systematic measurement errors occur at low moisture concentrations because the operator tends to cool the mirror past the frost point before the frost is detected, resulting in a measurement which is lower than the actual concentration. Additionally, frost point hygrometers tend to be large, expensive, and not readily adaptable to multi-sensor manufacturing environments. Likewise the output of a frost point hygrometer does not lend itself to integrated signal processing and automated measurements.
The subject invention overcomes these problems by making measurements at a single temperature with an AW device that can detect much smaller amounts of moisture on the surface of the device. This allows the subject invention to provide accurate measurements at very low concentrations of moisture with a minimum gas flow, on the order of 10 ml/min. The present invention, as described, is inherently small, cost effective, and readily adaptable to automation and signal processing.
The detection of trace contaminants such as moisture in inert gases using AW sensor systems has been widely demonstrated in prior art. AW devices in general and Surface Acoustic Wave (SAW) devices more specifically are described in Muller et al. U.S. Pat. No. 4,361,026, Martin et al. U.S. Pat. No. 5,235,235, Frye et al. U.S. Pat. No. 4,947,677, Fletcher U.S. Pat. No. 4,055,072, and Wohltjen U.S. Pat. No. 4,312,228 and are incorporated herein by reference in their entirety.